Light emitting diodes (LEDs), also referred to as LED lamps or LED light sources, are being more widely used in a variety of applications, such as traffic signals and signage. Thus, LED light sources are expected to replace incandescent, halogen, and fluorescent lamps in many general illumination applications within a few years.
Several factors motivate replacement of LED light sources with incandescent lamps, also referred to as incandescent bulbs or light bulbs, including improvements in reliability, power consumption, heat generation, vibration resistance, and operating lifetime.
However, LED light sources and incandescent lamps have different electric and dimming properties, which make the replacement difficult.
Thus, connecting LED light sources to conventional phase-cut dimmers, operating for example in Pulse Width Modulation (PWM) mode and normally designed to be used with incandescent lamps, can cause problems due to the fact that the light output or intensity of these incandescent lamps is not constant during the 50/60 Hz phase, whereas LED light sources that normally use a constant current driver provide a constant light output. Indeed, this problem is related to the exponential behavior of the incandescent dimming curve.
As illustrated in FIG. 1, the dimming curve can be defined as the relation between the position of the dimmer knob of the conventional phase-cut dimmer (axis of abscissa) and the light output of the corresponding lamp (axis of ordinates). Thus, FIG. 1 shows that the dimming curve of the LED light source has a linear shape within the operating dim range between 15% and 85%, and that of the incandescent lamp has an exponential shape within this range at dimming levels below 60%, i.e. 0.6.
Yet, the human eye is a photoreceptor that responds logarithmically to a change in light intensity, such that for an exponential change of light intensity of a given amount, the eye perceives a change of a linear amount.
Thus, the exponential behavior of the incandescent dimming curve is rather desired with respect to the linear behavior of the LED light source dimming curve.